The deadly H5N1 avian flu virus has entered Nigeria multiple times since it was first reported in February, according to a study published this week in the journal Nature. Lead author Claude Muller, with Luxembourg's National Public Health Laboratory Institute of Immunology, says H5N1 samples taken from two poultry farms were different from each other, and from the first strain that appeared in the country. "This means that these viruses were not directly related to each other, but probably introduced - the three of them - by independent routes."

The genetic differences suggest that the virus did not spread from farm to farm, but arrived separately, from either unprotected trade from Russia or Europe, or from migrating birds. Muller says with documented cases of the deadly virus in 14 of Nigeria's 31 states, bird flu remains both a public health and an economic threat. "It is important for the nutrition of the people. Chicken in principle could be an inexpensive source of high quality protein. It provides lots of secure jobs to a population that is in desperate need of such jobs."

The risks to poultry farming - Nigeria's second most important industry, after oil production - are greatest where farms are clustered, especially in the southwest poultry hub of the country. Veterinary officials at the University of Ibadan, Ibadan, Nigeria say that measures taken to control the virus -- such as quarantine and culling -- seem to be working better today than they did earlier this year, when the virus was first detected and spread rapidly. "I think the good news is that this does not seem to be a failure of the bio-security for the viruses that we have investigated," says Claude Muller. "But the bad news is that if there has been independent introduction, then of course there can be more independent introduction. And if this is a source that cannot be controlled, then this is really bad news."

Muller recommends continued testing of poultry operations to determine whether the H5N1 is present and if the strains are genetically related or not.

Nigeria has not reported any human case of the deadly virus. The virus must mutate in humans in order to spread from person to person. Such a mutation could unleash a global flu pandemic.